Brushless D.C. motors are typically provided with electrical circuitry which provides the drive electronics and the speed control means therefor. The drive electronics provides the electronic means which replaces the brushes and commutators of conventional D.C. motors. The speed control means is typically an optical tachometer which senses the speed of rotation of the drive shaft of the motor and applies signals to the drive electronics to control the speed of the shaft. The optical device of the tachometer also acts as a sensor for sensing the position of the rotar for communication of the currents to the motor windings.
In the past, the drive electronics and speed control circuits have been mounted in a "black box" which is separate and apart from the motor. This "black box" takes up additional space and requires a cable connecting it with the magnetic means, such as the field coils, of the motor.
The use of such a "black box" in conventional D.C. motors cannot be tolerated in certain applications because of space limitations. For instance, a disk drive unit is being made as small as practical and must be as compact as possible yet must have all of the features for driving magnetic disks as are found in larger disk drive units. Brushless D.C. motors are suitable for this application because of their reliability, absence of arcing, such as occurs at the brushes of conventional D.C. motors, and the ease of operation when coupled to a hub carrying a pair of spaced magnetic disks. Use of a conventional brushless D.C. motor for this application is not satisfactory because space would have to be provided for its "black box". This would in all likelihood be an inconvenience for the user of the disk drive unit. Because of the foregoing drawbacks, a need has arisen for an improved brushless D.C. motor, one in which the "black box" of the type described is eliminated.